Water absorbent resin (super absorbent polymer [SAP]) is a water-swellable, water-insoluble polymer gelatinizer. Water absorbent resin is capable of absorbing a water-based liquid in such a large amount as corresponding to several times to several hundreds of times the self weight. Water absorbent resin thus finds a wide range of applications, including (i) sanitary products such as disposable diapers, sanitary napkins, and incontinence pads, (ii) agricultural and horticultural water retaining agents, (iii) industrial waterproofing agents, and the like.
For such water absorbent resin, there have been proposed many monomers and hydrophilic polymers as raw materials. Industrially most common among different water absorbent resins is a polyacrylic acid (salt)-based water absorbent resin containing acrylic acid and/or a salt thereof as a monomer for high water absorbing ability.
The water absorbent resin is required to have various functions (excellent physical properties), as disposable diapers, which is a main application of the water absorbent resin, have increasingly higher performance. Specifically, the water absorbent resin is required to have, in addition to basic physical properties such as water absorption capacity without load and water absorption capacity under load, improved physical properties such as gel strength, water-soluble component, moisture content, water absorbing speed, liquid permeability, particle size distribution, urine resistance, antibacterial property, damage resistance, powder fluidity, deodorant property, anti-coloring property, low dustiness, and low monomer residual. There have been proposed various techniques for improving the above physical properties. Specifically, Patent Literatures 1 to 13 listed below each disclose a technique such as changing surface crosslinking, additive, or production process.
The techniques proposed include, in particular, techniques of surface-crosslinking water absorbent resin such as a technique of controlling a relative humidity and/or dew point of an atmosphere during a heat treatment to adjust the moisture in water absorbent resin (Patent Literatures 1 and 2), a technique in which a device for cooling water absorbent resin after a heat treatment has a defined size (Patent Literature 3), a technique of adding a water-based liquid to water absorbent resin during a heat treatment (Patent Literature 4), and a technique of carrying out a surface-crosslinking reaction twice (Patent Literature 5).
The techniques proposed further include methods for modification after surface crosslinking such as a technique of adding water to surface-crosslinked water absorbent resin for granulation (Patent Literature 6), a technique of adding a water-based liquid to heat-treated water absorbent resin when cooling the water absorbent resin for granulation (Patent Literature 7), a technique of adding a water-based liquid to heat-treated water absorbent resin for improved damage resistance (Patent Literature 8), a technique of adjusting the moisture content of a product for improved damage resistance (Patent Literature 9), a technique of humidifying and mixing water absorbent resin again after a heat treatment to increase the moisture content of the water absorbent resin (Patent Literature 10), a technique of adding a polyhydric metal salt aqueous solution to surface-crosslinked water absorbent resin (Patent Literatures 11 and 12), and a technique of adding an alkanolamine aqueous solution to surface-crosslinked water absorbent resin (Patent Literature 13).
Water absorbent resin produced with use of a technique mentioned above such as a surface-crosslinking technique, however, may be subjected to a mechanical damage during a step such as a conveying step or a filling step or even while a user, for example, processes the water absorbent resin into an absorbent article such as a disposable diaper. Such a mechanical damage, for example, destroys a surface-crosslinked layer of the water absorbent resin or even particles thereof, which has led to generation of dust or degradation of the physical properties.
The above phenomenon is a problem related to damage resistance of water absorbent resin. To solve this problem, there has been proposed a technique of adding water or an aqueous solution to surface-crosslinked water absorbent resin (Patent Literatures 9 to 13).